The present invention relates to a system for controlling a shifting in an automatic transmission and, more particularly, to a system for controlling a shifting of an automatic transmission which can be set both at main gear stages to be used for ordinary runs and at intermediate stages having gear ratios intermediate between those of the main gear stages.
As is well known in the art, a shift in the automatic transmission is effected on the basis of the data relating to an engine load such as the throttle opening and the data relating to a vehicle speed. The automatic transmission is shifted down, if the engine load increases or if the vehicle speed drops, but is shifted up if the engine load decreases or if the vehicle speed rises. The gear ratios of gear stages to be set are given relations close to the geometric series so as to improve the power performances so that the driving forces (or torques) to be attained at the individual gear states differ to some extents. As a result, if the driving force is insufficient for the running resistance, for example, the accelerator pedal is depressed to establish the necessary driving force, thus causing the downshift. If the driving force is excessive, on the contrary, the accelerator pedal is released to reduce the throttle opening, thus causing the upshift.
For a running resistance at a certain vehicle speed, however, there may occur a case, in which the driving force is insufficient at some gear stage but excessive at a one-step lower gear stage. This case is exemplified by point P appearing in the diagram of running performance of FIG. 6. In this state, the driving force to be generated at the 3rd speed is excessive for the running resistance in the state of a vehicle speed V.sub.1, but the driving force to be generated at the 4th speed is insufficient. If, in this case, the vehicle speed rises while the vehicle is running at the 3rd speed, the automatic transmission is shifted up to the 4th speed. At this 4th speed, the driving force is insufficient so that the automatic transmission is shifted down to the 3rd speed either as the vehicle speed drops or as the driver depresses the accelerator pedal. This so-called "busy shifting", in which the shifts are repeated, will deteriorate the riding comfortableness. Therefore, the shift control system, as disclosed in Japanese Patent Laid-Open No. 103042/1986, is constructed to inhibit any upshift if the shift frequency exceeds a predetermined value.
According to the system, as disclosed, the busy shifting is not caused because the shift itself is inhibited. Since, however, the gear stage is locked at a lower side for the running state, the engine speed is so disadvantageously increased that the mileage is degraded or that the noises grow serious.
According to another method for preventing the busy shifting, as disclosed in Japanese Patent Laid-Open No. 101549/1988, the acceleration after an up-shift is arithmetically determined to execute the up-shift, if the upshift is decided when the acceleration could be kept, or when the acceleration could not, the present gear stage would be hold.
According to this method of Japanese Patent Laid-Open No. 101549/1988, however, the upshift might be inhibited even if the engine has a driving force so sufficient for the running state that the engine speed increases to some extent. Since the run at the high speed of the engine is continued in that case, the method is accompanied by a problem that the mileage is degraded.